1. Field of the Invention
The present invention relates to a color picture tube having an in-line electron gun, and more particularly, to a color picture tube having an in-line electron gun for high-resolution display with a neck having a minimized diameter.
2. Description of the Related Art
Generally, the resolution of the color picture display screen depends largely on the spot size of the electron beam. Therefore, the spot size of the electron beam should be minimized to realize a high-resolution display.
The color picture tube is usually provided with an in-line electron gun in which three cathodes are arranged in a horizontal line to excite three different phosphors that produce the three primary colors, red (R), green (G), and blue (B). The in-line electron gun emits a plurality of electron beams to produce the colored image on the phosphor screen.
The in-line electron gun is mounted within the neck of the color picture tube and includes a series of electrodes which focuses and accelerates the electron beams. The electrodes each are provided with beam-guide holes for passing the electron beams R, G and B. A deflection yoke is mounted around a funnel of the color picture tube to scan the electron beams emitted from the electron gun across the screen of the panel.
The electrodes each have one or three beam-guide holes for passing the three electron beams.
The beam spot size Dt of the in-line electron gun is given by D.sub.t =.sqroot.((D.sub.x +D.sub.sa).sup.2 +D.sub.sc.sup.2) where D.sub.x is the diameter of the beam spot magnified by the main lens; D.sub.sa is the diameter of the beam spot formed by the spherical aberration of the main lens, and is defined by D.sub.sa =0.5.times.(MC.sub.so .alpha..sub.o.sup.3), where M is the magnification ratio of the main lens, .alpha..sub.o is the incidence angle of the main lens, and C.sub.so is the coefficient of the spherical aberration; and D.sub.sc is the diameter of the beam spot formed by repulsion of the charges of electrons.
Accordingly, the spot size of the electron beam is reduced as the spherical aberration of the main lens is diminished. And a method of enlarging the beam-guide holes of the electrodes in the main lens has been employed in the past to diminish the spherical aberration of the main lens.
However, in that method, the size of the electrodes, as well as the diameter of the neck, is also enlarged requiring a large amount of current for the deflection coils of the deflection yoke to scan the electron beams across the screen. Furthermore, the power consumption, as well as the amount of magnetic field leakage, increases during the deflecting operation of the deflection yoke. Thus, a separate electron beam shielding device is also needed resulting in increased production costs.